Inkjet printers operate a plurality of inkjets in each printhead to eject liquid ink onto an image receiving member. The ink can be stored in reservoirs that are located within cartridges installed in the printer. Such ink can be aqueous ink or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the image receiving member. The image receiving member is, for example, a print medium such as paper or an indirect image receiving surface such as a belt or drum that receives ink for later transfer to a print medium.
A typical inkjet printer uses one or more printheads with each printhead containing an array of individual nozzles through which drops of ink are ejected by inkjets across an open gap to an image receiving member to form an ink image. The image receiving member can be a continuous web of recording media, a series of media sheets, or the image receiving member can be a rotating surface, such as a print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force in a transfix nip formed by the rotating surface and a transfix roller. An inkjet printhead typically includes a plurality of inkjet ejectors in which each inkjet ejects drops of ink onto an image receiving surface. A print engine in an inkjet printer processes image data to control the operation of individual inkjets in one or more printheads to form printed ink images on the image receiving surface.
In order for the printed images to correspond closely to the image data, both in terms of fidelity to the image objects and the colors represented by the image data, the printheads are registered with reference to the imaging surface and with the other printheads in the printer. Registration of printheads refers to a process in which the printheads are operated to eject ink in a known pattern and then the printed image of the ejected ink is analyzed to determine the relative positions of the printheads with reference to the imaging surface and with reference to the other printheads in the printer.
In existing systems, the printheads form printed test patterns with a comparatively large number of inkjets that are distributed across the face of the printhead in the cross-process direction. For example, in one embodiment a printhead operates 150 inkjets to form a printed test pattern that is used to identify the registration of the printhead. In many configurations, however, only a small portion of the inkjets in a printhead eject ink drops onto an image receiving surface. For example, in a direct continuous web printer configuration, a media web, such as an elongated roll of paper, passes the printheads in the print zone. The media path and the print zone accept media webs with different widths during different print jobs in the printer. In some duplex printer embodiments, a single media web passes through the print zone twice in tandem for first side printing with a first group of printheads and second side printing with a second group of printheads. In both configurations, some printheads only partially cover the width of the media web in the cross-process direction. In some configurations, only a single inkjet in a printhead is used to form the printed image. Existing printhead registration techniques that require a large number of inkjets in the printhead to form a printed test pattern are unable to perform printhead registration using the different configurations of the print medium. Additionally, even if the printhead is capable of printing onto the image receiving surface with all or a majority of the inkjets, existing image analysis techniques may still be susceptible to image data noise and misidentification of printed dashes that occur when inkjets operate only intermittently. Consequently, improvements to the printhead registration process that enable robust printhead registration using test patterns formed using a variable number of inkjets would be beneficial.